1. Field of the Invention
This invention relates to a steering system for a vehicle, and more particularly to a variable steering ratio steering system for a vehicle in which the steering ratio or the ratio of the turning angle of the dirigible wheels (generally the front wheels) to the steering angle (the turning angle of the steering wheel) is changed according to the vehicle speed.
2. Description of the Prior Art
In the steering device for a vehicle, rotation of the steering wheel is converted into a lateral movement of the tie rods by way of a steering gear device such as of rack-and-pinion mechanism, and the front wheels connected to the respective tie rods are turned right or left according to the direction of the lateral movement of the tie rods. Conventionally, the steering ratio is generally fixed. However, it is preferred from the viewpoint of running stability that the steering ratio be reduced during high speed travel of the vehicle. On the other hand, it is preferred that the steering ratio be larger when the vehicle speed is low in order to improve response of the vehicle orientation to the operation of the steering wheel or to facilitate driving into a garage or the like.
In Japanese Unexamined Patent Publication No. 58(1983)-224852, there is disclosed a "speed-responsive type steering system" in which the steering ratio is changed according to the vehicle speed. In this steering system, a variable speed gearing comprising a pair of variable pitch pulleys and a V-belt passed around the pulleys is inserted between the steering wheel and the steering shaft, and the pitch diameter of the driven side pulley is increased by a step motor as the vehicle speed increases. Thus, the rotation transmission ratio from the steering wheel to the steering shaft is reduced as the vehicle speed increases so that the steering ratio is reduced as the vehicle speed increases. However, the steering system is disadvantageous in that since the variable speed gearing including the variable pitch pulleys is bulky, the overall size of the system is enlarged, and that there is a possibility that operation of the steering wheel will not be precisely transmitted to the steering shaft or the front wheels due to slippage of the V-belt. Further, when trouble arises in the step motor or the system for controlling the step motor and the step motor becomes incapable of constraining the driven side pulley, the variable speed gearing is held in the neutral position in which the input rotation cannot be transmitted to the output side, whereby steering of the vehicle becomes impossible.
Further, in the speed-responsive steering system disclosed in the aforesaid Japanese unexamined patent publication, the steering ratio is changed linearly with respect to the vehicle speed. In other words, the steering ratio is changed at a fixed rate over the entire vehicle speed range. When the front wheels are turned while the vehicle is running, the vehicle body rolls or yaws due to lateral acceleration. The rolling and yawing of the vehicle body and the lateral acceleration the vehicle body experiences for a given turning angle of the wheels increase exponentially with increase in the vehicle speed. Accordingly, in the speed-responsive steering system in which the rate of change of the steering ratio with respect to the vehicle speed is fixed and the turning angle of the wheels changes by a fixed amount over the entire vehicle speed range when the vehicle speed changes with the steering wheel held at a given angular position, rolling or yawing of the vehicle body is larger in the high vehicle speed range than in the low vehicle speed range, thereby adversely affecting the steering performance of the vehicle especially during cornering at a high speed.
Further, the speed-responsive steering system gives rise to a problem that when the vehicle speed changes during cornering, the turning angle of the front wheels changes even when the steering wheel remains stationary, thereby adversely affecting the running stability. This phenomenon is significant when the steering wheel is turned by a large angle.